The importance of improving BNF in green-beans through inoculation with Rhizobium leguminosarum bv. phaseoli is established. The final purpose of this study is to reduce N-fertilizer in beans production. Initial objectives where rizobial strains selection. beginning with molecular characterization of rhizobia and relating plasmidial DNA with chemical characteristics of soils. First results show very similar plasmids patterns in different strains of Rhizobium. facilitating the strain selection. Correlation studies show significative "r" values (a=0,05) between soils characteristics like pH, salinity, nitrates, ammonium. and plasmidial DNA concentration.